1. Field of the Invention
The present invention relates to a lens assembly disposed in a portable image taking device, which is suitable for being installed in high resolution CCD type equipments, such as digital camera, computer and the like, and is used for image reading. The lens assembly not only can ensure a comparatively strong negative refractive power, but also can ensure a necessary back focus and a total length of a portable image taking device.
2. Description of the Prior Art
Since the image taking lens assembly has an increasingly wide range of application, particularly for the mobile phone, palm computer (PC), MP3, and etc, the image taking has become one of the necessary functions of these products.
Nowadays, a mobile phone is usually equipped with a solid image sensor and an image taking lens assembly, such as CCD sensor (charge coupled device) or CMOS sensor (Complementary Metal Oxide Semiconductor). To comply with the requirement of miniaturizing the solid image sensor and the image taking lens assembly as well as improving the performance thereof, four-piece lens assembly in the products needs to be miniaturized and its performance thereof also needs to be improved. Therefore, with the popularization of photographic mobile phone, the research and development of the mobile phone is becoming more and more urgent. Hence, how to develop a four-piece lens assembly more satisfying the user's requirements has become an important consideration of the manufacturer.
In the early days when the mobile phone was equipped with the imaging taking device, the resolution of the CCD image taking lens assembly is at most up to 110-300 K pixels, and its size is at most miniaturized to 1/7-¼ inch. This relatively low requirement of optical performance can be easily satisfied as along as the image taking lens assembly has 1-2 lenses. Meanwhile, the total length of the optical system should be controlled properly for matching the thickness of the mobile phone, and for ensuring that the expected back focus can be easily achieved.
However, the resolution of the CCD inside the mobile phone has also been being increased year by year to Mega-pixel, in order that the optical performance of the image taking lens assembly can be improved correspondingly, the conventional configuration of the lens assembly having 1-2 lenses is unable to achieve the required performance. Therefore, a high performance lens assembly should be made up of 4 pieces of lenses, it will be unable to achieve an expected total length of the optical system and to ensure a required back focus either. Furthermore, it is also unable to arrange the IR cut filter and similar optical components.
For example, JP Pat. No. 2003-255222 discloses an image taking lens assembly consisted of 4 pieces of lenses, which is used in the optimum optical system of the digital camera. However, if the use of small type CCD smaller than ¼ inch is set as a criterion, it is unsuitable for high level image taking lens assembly due to the back focus is shortened. If the back focus is ensured by increasing the size of the CCD lens assembly, the total length of the optical system will be lengthened. Due to the comparatively small and limited space of an ordinary mobile phone, the conventional configuration is impossible to satisfy the double requirement of quality and space.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.